A review of the prior art reveals several US patents that define the status of scanning spot laser ablation and or eye-tracking systems. Lin, in U.S. Pat. No. 5,520,679, describes a scanning laser system and method of beam placement to produce smooth ablated surfaces; no compensation for eye motion, saccadic or other, is disclosed nor is there any feedback mechanism for controlling the location of the beam. Frey et. al, in U.S. Pat. Nos. 5,632,742, 5,980,513 puts forth a LADAR based eye-tracking apparatus in conjunction with a scanning excimer beam ablation system. Knopp et. al, presents in U.S. Pat. No. 5,865,832 a 2-axis servo-controlled mirror for tracking eye movement. Hohla in U.S. Pat. No. 5,645,550 uses a semi-rigid marking structure placed on the periphery of the eye.
These referenced patents are all implemented in analog form. The Frey and Knopp patents utilize contrast differences of eye tissue alone without the use of any external marking means; Frey selects the iris and pupil boundary while Knopp uses the iris and sclera boundary as the reference means for determining lateral (transverse or radial) eye position variations. In Frey""s approach, the pupil must be maintained in the dilated state. Knopp, in utilizing PSD""s (position sensing devices), averages the contrast of several portions of the iris/sclera boundary. While both systems under ideal conditions are capable of high eye tracking accuracy, the ablation procedure can produce changes in contrast across the stromal surface and thereby degrade the tracking precision. The Hohla patent presents an externally applied aiming assistance means for reducing such contrast variations, but does not specifically disclose means for achieving adequate speed in the tracking of saccadic eye motions.
Recent work in the field of evaluating overall optical performance of the eye using wavefront techniques (for example Williams et. al. U.S. Pat. No. 6,199,986) have presented means for determining the distortions of corneal surface and the other optical elements of the eye in terms of mathematical functions know as Zernike polynomials. PRK has been performed using the topography information provided by such wavefront techniques with considerable success in patients having slight to moderate myopia. A high degree of precision is required in locating the laser beam on the cornea and in determining the duration of ablation to produce the desired customized corneal surface. Real time topography measurement would greatly facilitate such customized ablations by insuring that the desired corneal topography is achieved.
A prospective system for performing PRK is one where the desired anterior corneal surface is sculpted with such a minimum of trauma that the surface topography immediately after ablation remains permanent thereafter, that postoperative discomfort is negligible and that after the regrowth of the epithelium, the wound healing response results in no corneal haze and the acuity of vision is retina and diffraction limited only. It is intuitive that for each laser-ablating pulse, minimizing the amount of energy delivered will also minimize the amount of phononic shock, heating and other traumatic effects on the cornea. The intensity of the pulse, i.e. the energy per unit area, must be above a threshold value, typically 50 mJ/cm2 for 193 nm, in order to break the molecular bonds of the stromal cellular structure. At an intensity of about 150 to 200 mJ/cm2, the per pulse ablation depth for stromal tissue averages 0.25 xcexcm. In prior art, this relatively constant per-pulse ablation depth provides an a priori means for predicting the resultant ablation depth, such a procedure being necessary in the absence of a real time topographic measurement apparatus. Lower intensities that could minimize trauma would require a longer operation time and would result in a very uncertain prediction of tissue removal. Minimizing the cross-sectional area of the ablating laser beam also can result in reduced trauma and also reduce the cost of the laser because the per pulse energy can be lowered thereby requiring a smaller laser that may take the form of a solid state frequency-multiplied laser or a small excimer laser. The minimization of intensity/fluence and beam crosssectional area is limited by the need to perform the desired ablation quickly enough to avoid excessive stromal dehydration and patient stress. A solution to minimizing operating time would involve maximizing the laser pulse rate. Such an approach places an increasing demand on the bandwidth of the laser beam scanning system and the real time topography system.
In most existing PRK procedures, the corneal epithelium is removed by mechanical and/or chemical means in order to expose the stroma for laser ablation. Swelling of the cornea has been observed using such techniques. In Hohla (U.S. Pat. No. 6,090,100), a method for removal of the corneal epithelium via excimer laser is presented wherein a dye, which is absorbed by the epithelium and not by the stroma, fluoresces in the presence of excimer radiation to guide selective epithelium removal. However, the use of a broad laser beam subjects the overall cornea to the same shock trauma encountered in general broad beam PRK. A narrow scanning laser beam is desirable to minimize such trauma along with removing the need for the fluorescent dye for the procedure of epithelial ablation.
The present invention addresses the foregoing items and proposes to meet the goals outlined within the state of the art of existing technology.
The system and method of the present invention generally comprise:
A pulsed laser operating at a high pulse rate producing a narrow beam of ablating radiation having a wavelength in the region of 193 nm; this pulsed beam combined collinearly with a similarly narrow continuous laser beam, the combined beam then directed to a two-axis electromechanically-controlled tiltable mirror whereupon it is reflected to a parabolic mirror (paraboloid) which collects the combined beam paths reflected from the tiltable mirror and collimates them after which they are separated by a wavelength selective meansxe2x80x94the pulsed laser radiation scanning beam being directed normally to the surface of the ablatable object (anterior cornea of the eye) and the continuous laser beam directed to a two-dimensional photodetection device which generates feedback voltage signals to control the tiltable mirror,
an annular scleral mask with inscribed reference markings which is fitted over the eye, the mask leaving the cornea exposed to the collimated rays of pulsed radiation, the reference markings imaged by an objective lens onto a photodetector array of linear pixel elements, the photodetection signals used to control the tiltable mirror to compensate for translational and rotation eye movement; the mask being attached prior at the outset so that a wavefront means can be used to measure the optical distortions of the eye whereupon a corneal surface is calculated to correct the distortions;
a raster videokeratography topography system utilizing a portion of the excimer/ablating laser radiation to project a raster pattern on the semi-diffuse surface of the cornea undergoing photoablation, the diffusely reflected pattern then optically imaged onto a two dimensional photodetector whereupon it is digitized and the surface topography calculated;
the wavefront means utilizing the same components of the beam scanning system to produce the requisite collimated rays for retinal image mapping and optical analysis;
a control system performing the following functions: Sensing tiltable mirror position and through an analog high frequency loop and a digital mid/low-frequency loop, positioning the mirror to achieve beam positioning precision exceeding that of prior art; sensing the position of the cornea and adjusting the beam position to the desired precision within 0.0005 second; monitoring the topography a minimum of 5 maps per second to provide feedback control of the ablating beam;
the control system incorporating digital algorithms to perform the functions of: Computing the areas of the cornea to be ablated in a manner to optimize speed while minimizing trauma; interpreting a photo-electronically imaged raster to produce real time topography measurements; interpreting the eye motion sensor data to measure corneal radial translational deviations and corneal radial rotational deviations.
The present invention is principally distinguished from prior art in that it combines into a single stand-alone system the functions of wavefront analysis and small-spot scanning laser photoablation controlled by real-time topographic measurement. The wavefront analysis means produces an ideal target corneal topography which is used as a template to direct the ablation process, feedback controlled by real-time topographic measurement means, until the error between the template and ablated cornea is negligible.
Also distinguishing the present invention is the ability to employ a low-cost ablating laser, where the cost of the laser is lower not only because of the narrow beamxe2x80x94hence requiring lower pulse energyxe2x80x94but because there is no necessity for high beam-intensity uniformity or high pulse-to-pulse energy constancy. The ability to be able to monitor almost continuously the corneal topography changes during ablation and also direct the beam with great accuracy to a moving cornea thereby enabling immediate correction, allows for considerable tolerance in such laser properties.
The present invention relates to and expands upon two prior patents issued to myself. In U.S. Pat. No. 5,350,374 a broad excimer laser beam is segmented by a two-dimensional excimer light modulator means into a multiplicity of collimated narrow sub-beams each being directed to a corresponding sub-area of the cornea and coinciding with a grid point projected from a videokeratography system. The depth computed from the imaged position of each raster point then serves as a real-time feedback signal to control the associated ablating sub-beam on a pulse-by-pulse basis. A disadvantage in this prior art is the difficulty in fabricating the modulator means to be able to withstand the intensity of the broad laser beamxe2x80x94a problem further aggravated by the need of a very high pulse rate to avoid using more than one sub-beam per pulsexe2x80x94which would negate the sought-after advantages of the narrow beam scanning concept. The present invention attempts to obviate this limitation while retaining the real time topographic control concept embodied in the prior patent. In U.S. Pat. No. 6,024,449, a real-time videokeratography system is disclosed which utilizes a laser raster projector technique that minimizes topographic errors due to variations in the axial position of the corneal surface. The present invention""s preferred embodiment uses one of the alternate embodiments of the prior patent; specifically, a portion of the output of the pulsed excimer laser used for photoablation becomes the source for the projected raster pattern on the semi-diffuse surface of the ablatable object. Because the laser pulse duration is in the nanosecond range, each raster pattern projected on the object (corneal) surface and then imaged on the two-dimensional CCD, results in a topography measurement unaffected by saccadic eye movement; further, the short wavelength improves the resolution of the raster pattern relative to visible light and the relatively low spatial coherence of the excimer reduces the adverse effects of speckle.
In attempting to further minimize the trauma accompanying PRK, photoablative removal of the epithelium (transepithelial PRK) using the narrow-beam scanning technique is incorporated into the overall PRK procedure of the present invention. An inherent fluorescent spectral component that characterizes epithelial photoablation is filtered and imaged on a CCD. When this spectral component essentially vanishes over the area of the cornea, removal of the epithelium has been accomplished whereupon real-time topography controlled ablation of the stroma can proceed.
A preferred embodiment of the invention incorporates the following:
1. A low per pulse energy (5 to 10 milli-Joules) excimer or solid state laser capable of operating up to approximately 1000 pulses/sec at a wavelength and pulse duration optimizing corneal photoablation.
2. A 2-axis servo-controlled flat mirror kinematically designed such that a coaxial/collinear beam, comprised of continuous wave laser radiation in the visible band and the pulsed laser radiation in the ultraviolet, impinges at the pivot point of the mirror such that the mirror position directly above the pivot point is invariant as the mirror is tilted along either or both axes. The beam is then reflected to a parabolic mirror whose focal point is at the pivot point of the mirror. Then the paths of the beam reflected from the parabolic mirror will all be collimatedxe2x80x94parallel to one anotherxe2x80x94and deflected to a broadband polarizing beam splitter (BPBS) which transmits the pulsed 193 nm radiation component to the ablation target (anterior of cornea) while reflecting the continuous wave radiation to a 2-dimensional photodetecter, the latter taking the form of a specialized charge coupled device (CCD) called a charge injection device (CID). Because there is a one-to-one correspondence between the continuous wave laser beam position on the CID and the excimer laser beam on the radial/transverse direction of the ablation target, a means is obtained for determining, and thereby controlling, the position of the excimer laser beam on the cornea to within 5 xcexcmxe2x80x94although a small number, is readily obtainable using a 10 mm diameter uniformly-distributed array of some 500,000 pixels and applying simple pixel interpolation. The CID is essentially a CCD that reads out only selected pixels so that by using the voltages from the mirror position detectors (electret sensors) to select a small area of pixels (perhaps 5 by 5), the intensities of those pixels can be two-dimensionally interpolated to locate the beam with the desired accuracy. A programmable logic device (PLD) digitally computes the coordinates of the beam position within about 40 xcexcsec. whereupon the coordinates are converted into analog form and compensated by analog circuitry before being fed into the each of the mirror-actuating amplifiers for orthogonal negative feedback control of the tiltable mirror. Presently available PLDs can perform some 500 million floating point instructions per second, and it is this capability that, in conjunction with high speed CCD readout and analog to digital and digital to analog conversions that enable their incorporation to meet the real time control demands of the present invention.
3. An all-analog damping feedback loop using the electret sensors detecting tilt in each axis of the 2-axis mirror; these sensors"" output being amplified using low noise wide bandwidth operational amplifiers. A large value of negative derivative feedback critically damps out the resonances of the mechanical portion of the system, increases the bandwidth an order of magnitude above the dominant resonant frequency of the mechanical system, reduces non-linearities of the electromechanical system, and reduces the effects of external vibrational forces.
4. A high speed videokeratography system for measuring corneal topography utilizing the excimer laser as the light source for the raster pattern projection. Then by focussing this 193 nm wavelength diffuse light pattern on the cornea onto a back-illuminated charge coupled device (BCCD) sensitive to the 193 nm radiation, a topographic resolution of nearly a micron can be achieved. A digital algorithm generates instantaneous topography measurements over a grid of nearly two thousands points on a cornea undergoing ablation, enabling full topography surface measurements of at least 5 times a second over the course of the ablation procedure. Rapidity of measurement enables the use of averaging and regression techniques to maximize accuracy. This high speed also facilitates integrating the topography system into real-time feedback control of the ablation system.
5. A digital-analog (hybrid) eye tracking system enabling precise ablative beam positioning on the cornea regardless of positional variations such as characterized by the saccades of the in-vivo eye. The same lens used to image the raster pattern on the BCCD is also used to focus the corneal mask reference markings onto a photodetector constructed by a series of linear charge coupled devices to form what is herein referred to as a segmented charge coupled device (SCCD). Analog voltages corresponding to pixel intensities of the markings are converted to digital form, and by means of a digital algorithm, the instantaneous position (in x-y coordinates) in digital form is converted into two analog voltagesxe2x80x94orthogonally controlling the tiltable mirror, and after being phase compensated, each voltage is summed into the actuating amplifier controlling the tiltable mirror thereby insuring that the ablating beam is always directed to the desired position on the cornea of the eye.
6. Digital algorithms, written to minimize execution times of the a) beam detection/positioning algorithms, b) topography calculation algorithms.
7. A wavefront analysis means integrated within the apparatus of the invention whereby the continuous beam laser is used to direct the same collimated beams (absent the pulsed laser component) to the pre-operative eye, with the resulting retinal spot images being imaged on the same CCD used for epithelial ablation. Because the eye-tracking means of the invention is activated, the absolute spot positions the retina can be very accurately located. Then in combination with corneal topography (using the existing system in conjunction with a fluorescent dye on the pre-operative eye) whereby the pre-operative localized corneal curvature at the point of corneal entry for each beam is calculated, the desired post-operative corneal curvature is determinedxe2x80x94i.e., the corneal topography required for perfect (diffraction limited) focus of the collimated rays on the retina.